As an image recording method of forming an image on a recording medium, for example, paper, based on image data signals, there are an electrophotographic system, a sublimation or melting thermal transfer system and an inkjet system. The electrophotographic system is a complex system and an apparatus therefor is expensive because it requires such a process that an electrostatic latent image is formed on a photoreceptor drum through charge and exposure. The thermal transfer system involves a high running cost and generation of waste materials due to the use of an ink ribbon, although an apparatus therefor itself is inexpensive. In the inkjet system, on the other hand, image formation is carried out with an inexpensive apparatus in such a manner that an ink is directly ejected to only a necessary image area on a recording medium, and thus the ink can be used efficiently to reduce the running cost. Further, the inkjet system causes less noise, and thus it is excellent as the image recording method.
The inkjet recording system includes, for example, a system of flying ink droplets by pressure of vapor generated by heat from a heat generator, a system of flying ink droplets by mechanical pressure pulses generated by a piezoelectric element, and a system of flying ink droplets containing charged particles by utilizing an electrostatic field (refer to Patent Document 1 and Patent Document 2). The system of flying ink droplets with vapor or mechanical pressure cannot control a flying direction of ink droplet, and there are some cases where ink droplet is difficult to be accurately reached to the desired position on a printing medium due to distortion of ink nozzle and air convection.
On the contrary, the system utilizing an electrostatic field controls the flying direction of ink droplet with the electrostatic field to enable ink droplet to be accurately reached the desired position, and thus it is advantageous in that an imaged material (printed material) with high image quality can be produced.
As an ink composition for use in the inkjet recording system utilizing an electrostatic field, an ink composition comprising a dispersion medium and charged particles containing at least a colorant is ordinarily employed (refer to Patent Document 3 and Patent Document 4). The ink composition containing a colorant can form inks of four colors, i.e., yellow, magenta, cyan and black, by changing the colorant, and can also form special color inks of gold and silver. Accordingly, the ink composition is useful for producing a color imaged material (printed material). Heretofore, however, it is difficult to eject ink droplets constantly and stably in the inkjet recording for a long period of time.
Patent Document 1: U.S. Pat. No. 6,158,844
Patent Document 2: U.S. Pat. No. 3,315,334
Patent Document 3: U.S. Pat. No. 5,952,048
Patent Document 4: JP-A-8-291267 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
In the inkjet recording system in which charged particles are ejected utilizing an electrostatic field, in general, since the charged particles is concentrated and ejected by electrostatic force, concentration of the charged particles after the ejection is reduced in comparison with concentration of the charged particles in the ink composition before the ejection. Accordingly, when only an ink composition initially fed (initial feeding ink composition) has been continuously used, the concentration of the charged particles decreases and as a result, electric conductivity of the ink composition necessary for the ejection decreases to cause a problem in that the ejection of ink cannot be conducted. Also, in the case wherein concentration of charged particles in the initial feeding ink composition is same as concentration of the charged particles in a replenishing ink composition, the reduction in the concentration of charged particles due to the concentration and ejection is unable to compensate and consequently a problem arises in that density of the image formed is reduced.
Further, in the inkjet recording system in which charged particles are ejected utilizing an electrostatic field, as a charge amount of the charged particle increases, stronger electrostatic forth acts on the particle so that the particle is preferentially ejected. Also, since the generation of charge on particle is caused by adsorption of a charge controlling agent on the surface of particle, as the surface area of particle increases, the charge amount increases. Specifically, as a diameter of the particle increases, the charge amount increases. Accordingly, as the diameter of the particle increases, stronger electrostatic forth acts on the particle so that the particle is preferentially ejected. In fact, however, the diameter of the charged particle in the ink composition is not simple but has distribution. Since the particles having a large diameter are preferentially ejected as described above, the particles having a small diameter are not ejected and accumulated in the ink composition during a large amount of inkjet recording and as a result, a problem occurs in that an average particle diameter of the ink composition decreases. Moreover, since the electrostatic forth acts on the particles having a small diameter is weak, they cause problems in that response to pulse frequency for driving the ejection decreases and in that density of the image formed is reduced.